1. Field of the Invention
The present invention relates to a channel estimation method, and more particularly, to an adaptive channel estimation method applied in a multicarrier communication system.
2. Description of the Prior Art
In a conventional communication system, the communication band is usually divided into a plurality of subchannels to increase the utilization efficiency of the band. Each of the subchannels corresponds to a specific subcarrier. This kind of communication systems is regarded as “multicarrier communication systems”.
In order to correctly perform equalization to the signals received by a receiver in a multicarrier communication system, the receiver has to execute channel estimation for obtaining channel responses. Take the DVB-T (digital video broadcasting-terrestrial) standard for example. A transmitter may transmit signals called symbols including pilot signals that can be identified by a receiver such that the receiver can execute channel estimation to the subchannels having the pilot signals and thereby obtain the corresponding channel responses. Moreover, the receiver can utilize these corresponding channel responses, so as to determine channel responses of the other subchannels through interpolation.
FIG. 1 is a diagram illustrating how pilot signals are allocated in a few symbols according to the DVB-T standard. In this diagram, each horizontal row represents a symbol; each vertical column represents a subchannel. In FIG. 1, the subchannels can be divided into three different types according to the allocated way of pilot signals. A first type of subchannel is the “continual pilot signal subchannel”, which is dedicated for transmitting only pilot signals. A second type of subchannel is the “scattered pilot signal subchannel”. In each scattered pilot signal subchannel, a pilot signal is transmitted every four symbols, and data signals are transmitted in the other three symbols. A third type of subchannel is “simple data signal subchannel”, which is dedicated for transmitting only data signals. Accordingly, pilot signals are allocated in continual pilot signal subchannels and scattered pilot signal subchannels, and data signals are allocated in scattered pilot signal subchannels and simple data signal subchannels. For more information about the DVB-T standard please refer to the following paper: “Digital Video Broadcasting (DVB); Framimg structure, channel coding and modulation for digital terresterial television”, ETSI EN 300 744 V1.4.1, European Broadcasting Union (EBU), 2001-01.
In a conventional communication system, the channel estimation process is typically divided into two parts. The first part includes utilizing the pilot signals of two consecutive symbols and thus determining channel responses of continual pilot signal subchannels and scattered pilot signal subchannels through linear interpolation. The second part includes utilizing the channel responses determined in the first part, so as to determine channel responses of simple data signal subchannels through interpolation by using a multi-tap filter with fixed coefficients.
However, during the first part of the channel estimation process, the channel response of the subchannel corresponding to the scattered pilot signal of a preceding symbol is obtained according to the channel estimation results determined according to the scattered pilot signals of the preceding symbol and the current symbol. Since pilot signals are scattered in scattered pilot signal subchannels, the linear interpolation mentioned in the first part consumes a lot of memory space and thus causes delay during making decision. Additionally, when a subchannel requires strict frequency selection, a filter with more taps is needed to get accurate interpolation results during the second part of the channel estimation process.